The present invention relates to a visual display apparatus and, more particularly, to a visual display apparatus, for example, a head-mounted display apparatus, which enables an image to be observed at a wide field angle and high resolution.
Helmet- and goggle-type head-mounted display apparatuses have heretofore been developed for the purpose of enabling the user to enjoy virtual reality or wide-screen images by oneself.
Incidentally, a conventional display apparatus designed to be mounted on the observer's head is known in which, as shown in FIG. 38, an image displayed on a two-dimensional display device 1, e.g., a liquid crystal display device, is reflected by a half-mirror 2 which is disposed in front of a user's eyeball at a tilt of 45.degree. to the user's visual axis, and the reflected image is enlarged by a concave mirror 3 which is disposed forwardly of the half-mirror 2, and then the enlarged image is passed through the half-mirror 2, thereby enabling the user to observe the enlarged displayed image (see Japanese Patent Application Laid-Open (KOKAI) No. 3-188777). It should be noted that the two-dimensional display device 1 is disposed in the vicinity of the front focal point of the concave mirror 3.
Regarding the optical system shown in FIG. 38, let us consider the pass area of the chief ray on the assumption that the distance (working distance) between the above-described optical system and the user's eyeball is WD, the diameter of the concave mirror 3 is D, and the thickness of the optical system is t. On the above assumption, the following expression holds: ##EQU1##
It should be noted that D and t are approximately equal to the vertical and horizontal (as viewed in the figure) widths, respectively, of the half-mirror 2 tilted at 45.degree.. Consequently, D ant t are approximately the same.
In order to enable the conventional head-mounted display apparatus to be conveniently used, WD must be set in the range of from 10 mm to 25 mm. In a case where WD is 20 mm and the field angle .theta. is 40.degree., for example, it will be understood from the above expression that the thickness t of the optical system becomes considerably large, i.e., t=53 mm. Moreover, it is also necessary to ensure a pass area for subsidiary rays in addition to that for the chief ray in actual practice. Therefore, it is necessary to further increase the size of the optical system.
In addition, since a small-sized two-dimensional display device does not have a sufficiently large number of pixels, when an image that is displayed on such a small-sized two-dimensional display device is enlarged for observation, satisfactorily high resolution cannot be obtained. Under these circumstances, the present inventor considered arranging a plurality of such display devices in side-by-side relation to obtain a wide screen. However, a lightweight liquid crystal display device is generally used for a display apparatus of the type which is mounted on the user's head as in the case of a head-mounted display apparatus, and as shown in the front view of FIG. 39, such a liquid crystal display device has a display screen G surrounded by the peripheral portion of a substrate K. Therefore, when a plurality of such display devices are arranged in side-by-side relation, the substrate portions K of the adjacent display devices interfere with each other, thus giving rise to a problem.
Next, U.S. Pat. Nos. 5,198,928, 4,743,964 and 5,357,277 are related art using two CR displays or LCDs (Liquid Crystal Displays) as two-dimensional display devices. However, in any of the prior art, two 2-dimensional display devices are provided for observation with the left and right eyes. Accordingly, the right eye sees only the image of the two-dimensional display device for the right eye, and the left eye sees only the image of the two-dimensional display device for the left eye. In other words, the image of only one two-dimensional display device is led to one eye.
The U.S. Pat. No. 5,408,346 is related art in which the images of two 2-dimensional display devices are led to one eye. The prior art discloses an arrangement in which two 2-dimensional display devices are superimposed on one another to double resolution (see FIG. 1 of U.S. Pat. No. 5,408,346), and another arrangement in which only the image of one two-dimensional display device is projected as an enlarged image, and the image of the other two-dimensional display device is not enlarged but superimposed on the inner portion of the enlarged projected image of the first-mentioned two-dimensional display device, thereby enabling the inner portion of the projected image to have high resolution (see FIG. 2 of U.S. Pat. No. 5,408,346). However, neither of the arrangements contain an arrangement in which two 2-dimensional display devices are adapted to respectively display the images of approximately halves of an image to be viewed by the observer. Accordingly, if it is intended to show an image of wide field angle to the observer, the sizes of the two-dimensional display devices and the associated ocular optical system increase correspondingly, making it impossible to realize a compact structure.